In light of the ever more stringent emissions regulations that are planned to take effect over the next few years, including California Low Emission Vehicle II (LEV II), Federal USA EPA Tier 2, and European Union EU-IV and EU-V, pre-catalyst engine-out hydrocarbon (HC) emissions, especially during cold start and warm-up, are attracting significant efforts in research and development. This is due, in large part, to the fact that as much as 80 percent of the total hydrocarbon emissions produced by a typical, modern, light-duty vehicle during the Federal Test Procedure (FTP) can occur during the first 120 seconds of the test.
These high levels of emissions are largely attributable to cold engine and exhaust component temperatures. Specifically, cold engine components necessitate fuel-rich operation, in which the excess fuel is used to compensate for the portion of fuel that has attached to the walls of the intake system and combustion chamber and, thus, is not readily combusted. In addition, a cold three-way catalyst cannot reduce a significant amount of the unburned hydrocarbons that pass through the engine during cold-start. As a result, high concentrations of unburned hydrocarbons are emitted from the tailpipe. It is understood in the art that the over-fueling associated with excessive hydrocarbon emissions during cold-start could be considerably reduced through the use of gasoline vapor rather than liquid gasoline.
A variety of systems have been devised to supply fine liquid fuel droplets and air to internal combustion engines that work relatively well after engine warm-up. These systems either supply fuel directly into the combustion chamber (direct injection) or utilize carburetor(s) or fuel injector(s) to supply the mixture through an intake manifold into a combustion chamber (indirect injection). In currently employed fuel injected systems, the fuel-air mixture is produced by atomizing a liquid fuel and supplying it as fine droplets into an air stream.
In conventional spark-ignited engines employing port-fuel injection, the injected fuel is vaporized by directing the liquid fuel droplets at hot components in the intake port or manifold. Under normal operating conditions, the liquid fuel forms a film on the surfaces of the hot components and is subsequently vaporized. The mixture of vaporized fuel and intake air is then drawn into the cylinder by the pressure differential created as the intake valve opens and the piston moves towards bottom dead center. To ensure a degree of control that is compatible with modern engines, this vaporizing technique is typically optimized by controlling the injection timing such that the atomized liquid fuel is injected once per engine cycle for each cylinder and timed such that fuel is injected when the engine intake valve of that cylinder is closed, not open. This fuel injection firing strategy, synchronous, closed-valve, sequential, single fire, fuel injection, has become the preferred practice for automotive applications. Nonetheless, several other injection firing strategies have been developed over the years as outlined by the Society of Automotive Engineering—Surface Vehicle Recommended Practice SAE-J1832 (Rev. February 2001) “Low Pressure Gasoline Fuel Injector.” These alternative fuel injection firing strategies include: simultaneous double fire, alternating double fire, simultaneous single fire, sequential single fire, and make-up fueling.
However, sequential single firing has become the preferred approach for very good reasons. First, the timing can be chosen for each cylinder so that injection only occurs when the engine intake valve of that cylinder is closed which eliminates the risk of having liquid fuel spray entering the cylinder. Second, the number of injection events per engine cycle is limited to one. If the injection is broken up into several repeated injections in every engine cycle, the injectors have to be cycled at shorter pulse widths (i.e., shorter duration for each injection) to keep the fuel flow the same as for one single injection event per engine cycle. This is especially critical at low engine loads such as engine idling since where fuel required is relatively low. When metering liquid fuel, the injectors have to be cycled at very short pulse widths if injection occurs several times per engine cycle. From a dynamic standpoint, it is preferred not to operate injectors at very short pulse widths, e.g., less than about 1 ms, since all injectors tend to be non-linear when very short pulses are used (i.e. the amount of fuel delivered does not scale linearly with the pulse width at very short pulse width durations). Operating a fuel injector in the non-linear region tends to make fuel control less precise and more difficult.
In addition to the different injection firing strategies outlined above, a variety of techniques have been proposed to address the issue of fuel control and mixture preparation especially during the critical cold-start phase when mixture preparation is the most challenging.
U.S. Patents proposing fuel injection metering and firing techniques include U.S. Pat. No. 4,091,773, issued to Gunda, and U.S. Pat. No. 4,096,833, issued to Sweet, both of which disclose an asynchronous, pulse width modulated, multiple fire several times per engine cycle, fuel metering technique for single point fuel injection systems, wherein fuel is injected at a single point for multiple cylinders upstream of the fuel charge intake for the engine. Both temporal (engine cycle-to-cycle) and spatial (engine cylinder-to-cylinder) fuel stratification and variability are known challenges for single point fuel injection system and they have not been adopted. Also, this does not address the problem of reducing production of excess hydrocarbons on cold start-up.
U.S. Pat. No. 4,269,157, issued to Fujishiro, also proposes an asynchronous multiple fire fuel metering technique for single point fuel injection systems. It is further proposed to use frequency modulation as opposed to pulse width modulation to meter the fuel mass flow. As well as using the now less preferred single point injection approach, this does not address the problem of reducing production of excess hydrocarbons on cold start-up.
U.S. Pat. No. 4,724,816, issued to Kanno et al., discloses a synchronous pulse width modulation strategy for multi-point fuel injection systems that transitions between single injection firing and multiple injection firing depending on the amount of fuel required. It is said that, to deliver the amount of fuel required to start an engine with liquid fuel at very cold ambient conditions, the fuel injectors have to stay open for up to 13 times longer than under normal operating conditions (200 ms injection per engine cycle for cold start at very low temperatures compared to 15 ms injection per engine cycle at heavy engine load). This patent proposes using a sufficient number of short (15 ms or less) injection pulses to provide enough fuel for cold starts. The primary objective seems to be to save on the cost of electronics.
U.S. Pat. No. 5,076,238, issued to Rosenau et al., also proposes a pulse width modulation strategy for multi-point fuel injection systems that transitions between single injection firing for normal operating conditions and multiple injection firing during cold starts at very cold temperatures. It is claimed that by dividing the large amount of fuel needed for cold starts at very low temperatures into several equal fuel doses per engine cycle, the ability to start with liquid fuel at very low temperatures can be improved.
U.S. Pat. No. 6,085,718, issued to Nishimura et al., describes performing fuel injection in a specific pattern to provide a lean fuel mixture during cold start. This method delivers fuel in two steps with early and late split injections during the intake stroke.
U.S. Pat. No. 6,367,452, issued to Shima et al., describes a system that uses a fuel injection pump, in addition to a feed pump. A fuel metering valve is used that varies its opening area via the amount of current supplied to its solenoid. This fuel metering valve controls the pressure of the high-pressure fuel being delivered from the fuel injection pump. This process is synchronous with the engine.
While the last four patents may provide some improvement, because they provide liquid fuel for cold starts, they do not completely address the problem of excess hydrocarbons.